Chemical reaction during catalyst preparation

Complexes containing one binap ligand per ruthenium (Fig. 3.5) turned out to be remarkably effective for a wide range of chemical processes of industrial importance. During the 1980s, such complexes were shown to be very effective, not only for the asymmetric hydrogenation of dehydroamino adds [42] - which previously was rhodium s domain - but also of allylic alcohols [77], unsaturated acids [78], cyclic enamides [79], and functionalized ketones [80, 81] - domains where rhodium complexes were not as effective. Table 3.2 (entries 3-5) lists impressive TOF values and excellent ee-values for the products of such reactions. The catalysts were rapidly put to use in industry to prepare, for example, the perfume additive citronellol from geraniol (Table 3.2, entry 5) and alkaloids from cyclic enamides. These developments have been reviewed by Noyori and Takaya [82, 83]. 

Table 2 reports the catalytic activities of the catalysts prepared for 2.6-DTBP oxidation. All the titanium grafted materials were active as catalysts for liquid phase oxidation of 2.6-DTBP, and catalytic activity decreased in the order of MCM-48 (24.5% conversion) > HMS (22.8%) > KIT-1 (16.0%) > MCM-41 (14.3%) > SBA-1 (5%). Apparently. 3 dimensional channel system of MCM —48, and HMS with small particle size and textual mesoporosity proved to be useful in liquid phase reaction [1,2,3], Chemical analysis of the titanium-grafted SBA-1 by EDX showed far less titanium at the surface than the others it seems surface nature of SBA-1 synthesized in acidic medium is different from the rest. All Ti-grafted samples suffered from titanium leaching during the liquid phase oxidation HMS host resulted in over 4 % loss in metal content while the rest showed 2%. 

Over the past 10 years a multitude of new techniques has been developed to permit characterization of catalyst surfaces on the atomic scale. Low-energy electron diffraction (LEED) can determine the atomic surface structure of the topmost layer of the clean catalyst or of the adsorbed intermediate (7). Auger electron spectroscopy (2) (AES) and other electron spectroscopy techniques (X-ray photoelectron, ultraviolet photoelectron, electron loss spectroscopies, etc.) can be used to determine the chemical composition of the surface with the sensitivity of 1% of a monolayer (approximately 1013 atoms/cm2). In addition to qualitative and quantitative chemical analysis of the surface layer, electron spectroscopy can also be utilized to determine the valency of surface atoms and the nature of the surface chemical bond. These are static techniques, but by using a suitable apparatus, which will be described later, one can monitor the atomic structure and composition during catalytic reactions at low pressures (< 10-4 Torr). As a result, we can determine reaction rates and product distributions in catalytic surface reactions as a function of surface structure and surface chemical composition. These relations permit the exploration of the mechanistic details of catalysis on the molecular level to optimize catalyst preparation and to build new catalyst systems by employing the knowledge gained. 

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